The present invention is directed to the production of electrical devices, e.g. electrolytic capacitors and superconducting materials based on the use of valve metals such as tantalum and niobium and its alloys. In work done previously by the assignee of this patent application there have been described developments wherein filaments of refractory metals such as tantalum and niobium are extruded in a softer metal such a copper while encased in a surrounding sleeve of the valve metal e.g. tantalum or niobium. Utilizing this technology filaments of either tantalum or niobium have been drawn to very small diameters while in the copper matrix and while surrounded by the constraining layer of tantalum or niobium. The copper is leached out from between the very fine filaments in the final product while the fine filaments are constrained in a manageable bundle by the outer constraining acid resistant layer of tantalum or niobium. This technology is more fully described in our prior parent U.S. Pat. No. 5,869,196 and our PCT Application Serial No. PCT/US03/24724, filed Aug. 7, 2003.
In the present invention there is an improvement over the processed described in the '196 patent above wherein the constrained mass of small fine tantalum or niobium filaments is subjected to a rolling deformation which flattens the individual small filaments so that each wire has aspect ratio of at least 5:1. This relatively high aspect ratio in the final compressed and flattened individual fine filaments has the following major advantages. When the Nb is reacted to form Nb3Sn, currently, round Nb filaments require in excess of 180 hours at 700° C. to completely react 4-micron filaments. The same filament rolled to an aspect ratio of 5:1 will be only 1.26μ thick by 12.6μ wide and can be fully reacted using considerably shorter times. By reducing the time and distance for reaction, finer grain size and more importantly, uniform Nb3Sn are obtained and as a result, higher current densities as well.
A flat conductor is easier to wind into a magnet and it has higher filling factor where the void space when a round conductor is used is eliminated. Another important consideration is that thin conductors are more ductile and can be wound into magnets of small diameters. A reacted Nb3Sn; flattened to an aspect ratio of 5:1, with a thickness of 0.02 mm thickness by 0.02 mm wide can be bent around a radius of 5 cm without fracturing the wire. This offers the possibility of reaction and winding, similar to present day ductile NbTi magnets.
An unexpected discovery has been that the improvements for superconductor have been shown to be even more important for capacitors. The filament size, its number, spacing and surface area/unit volume are exactly the same critical designs parameters for both applications. In the case for superconductors, the entire Nb filament is converted to superconducting Nb3Sn. For capacitors, the surface of the Nb filament is converted to Nb205, the dielectric compound for capacitors use. In fact, with only minor size and shape adjustment, the wire can be used as either a superconductor or as an anode for solid Nb electrolytic capacitors. Confirmation of the dual use possibility of this invention are further demonstrated by two recent publications, Alternative Materials for Electrolvtic Capacitors by Dr. K. Reichert, T. I. C. Bulletin, No. 109 March 2002 page 2–3, and Powder for Capacitor, Sintered Body Thereof and Capacitor Using the Sinter Body PCT/JP01/10484 OMORI, Kazuhiro, 6 Jun. 2002, where Ta and Zr alloys of Nb, used for superconductors, has been shown to have excellent capacitor properties as well.